[libc++] Fix environment variable passing in libc++'s `SSHExecutor` `lit` utility

Summary:
Quote the value of environment variables when passing them to the SSH
client in SSHExecutor in libc++'s lit utilities. Without the quotes,
an environment variable like FOO="buzz bar" gets passed incorrectly
like this, ssh env FOO=buzz bar, which causes bar to be treated as a
command to run, not part of the environment variable value.

We ran into this when using SSHExecutor to do bringup of our CUDA
libcu++ port on an embedded aarch64 system.

Patch by Bryce Adelstein Lelbach.

Differential Revision: https://reviews.llvm.org/D65960

llvm-svn: 368317